1. Field of the Invention
This invention relates in general to roadside beacon systems, and more particularly to a novel roadside beacon system which is used for data transmission between a vehicle and road side antenna (beacon). The invention is also related to vehicle position calibration in a navigation system in which, after position data on a departure point are initialized, at least vehicle speed data and direction data are sensed, and the vehicle's position is calculated by using the data in order that the present position of the vehicle can be displayed.
2. Background of the Invention
A so-called "navigation system" has been proposed in the art in which a small computer and a display unit are installed on a vehicle. In such a navigation system, for example, road map data are read out of a memory device comprising a compact disc and so forth and a reproduction of a road map can thus be displayed on a display unit. Vehicle speed and moving direction are detected by a vehicle speed sensor and a direction sensor, respectively, in order to determine the current position and travel direction of the vehicle. According to these data thus determined, a symbol representing the vehicle is marked at the applicable position on the road map displayed on the display unit.
The use of the navigation system permits the operator of the vehicle to visually detect the current position and travelling direction of the vehicle so that he can drive the vehicle directly to the destination without losing his way.
However, such a kind of navigation system is disadvantageous in the following points. In the navigation system, errors inherent in the vehicle speed sensor and the direction sensor are accumulated as the vehicle travels without error corrections. When the vehicle has traveled for a certain distance, the vehicle position displayed on the display unit becomes much different from the true position of the vehicle. That is, the system does not work satisfactorily, and accordingly the operator may lose his way. It should be noted that the degree of error per unit is distance traveled may vary considerably from application to application, as the degree of error inherent in the vehicle speed sensor and direction sensor may fluctuate over time due to variations of the ambient conditions of these sensors.
In order to overcome the above-described difficulty, a so-called "roadside beacon system" has been employed. In the roadside beacon system, roadside antennas are installed alongside road networks at intervals of less than the distance with which the above-described accumulation of errors reaches a certain value. Each of the antennas thus installed is used to transmit a signal covering only a small area of the road. A signal which contains position data of the antenna or the place of correction, is thus transmitted and is received through a mobile antenna installed on the vehicle. Signal data is processed by a computer so that the position and travelling direction of the vehicle are calibrated according to the signal thus received.
With the roadside beacon system, the position and travelling direction of the vehicle can be displayed accurately with the accumulation of errors maintained less than a predetermined value, and as a result the navigation system works satisfactorily. Furthermore, the roadside beacon system is advantageous in that, if the roadside antennas are installed at positions near railways or railway crossings where the direction sensor is liable to operate with error because of strong magnetic fields caused by the railways, then errors attributed to external factors can be effectively corrected.
In the above-described roadside beacon system, a signal including the position data and road direction data is transmitted from each of the roadside antennas which have considerably high directivity. Therefore, the vehicle receives the signal only when passing through the area covered by the signal so that the necessary calibration is carried out according to the signal thus received. If the area covered by a signal is made larger in order to enable vehicles to receive more sufficient data, then the peak of the field strength of the radio wave transmitted from the roadside antenna is reduced as much, and it becomes difficult to detect the position correctly where the vehicle confronts the roadside antenna. That is, the conventional roadside beacon system suffers from drawbacks that when a data communication region is broadened, the signal receiving position with respect to the roadside antenna may be shifted so much that the satisfactory calibration effect cannot be obtained.
The fundamental function of the roadside beacon system is to transmit the signal including the position data and the road direction data to the vehicle having the navigation system. However, in order to improve the utility of the roadside beacon system, it is also desirable for the system to provide additional data or indicated function as follow:
(1) Traffic data on traffic congestion, construction work and other road use conditions around each of the roadside antennas are additionally applied to assist drivers in driving smoothly;
(2) Detailed map data including houses (with names) around the roadside antennas are added so as to facilitate the arrival of the vehicle at a desired destination;
(3) A relatively wide range of road map data including the areas where the roadside antennas are installed are additionally given to the navigation system to renew the road map on the display unit, thereby to allow the vehicle to smoothly travel to a far destination; and
(4) Devices for bi-directional communications between vehicles and the roadside antenna system are required so as to carry out stable burst data communications.
For this purpose, it is essential to widen the bandwidth of the signal radiated from the roadside antennas, and to broaden the area covered by the signal. However, the widening of the bandwidth of the signal and the broadening of the area covered by the signal causes another difficulty in that a vehicle position detected with respect to the position of the roadside antenna may be greatly deviated. As a result the calibration of the vehicle position, which is the original object of the system, thereby becomes erroneous, being affected by the deviation of the detected position.